Such laser arrangements are used for scientific purposes, on the one hand, and can be used in material processing, on the other hand, particularly if fine structures are to be produced.
In the mode-locked state, a laser emits laser pulses instead of a continuous laser light (continuous wave (cw) operation), by storing energy and emitting it in pulse-like manner. The duration of the periods of these pulses will generally correspond to the round trip time of the pulses in the laser resonator, and, e.g., with a length of the linear resonator of 2 m, pulses with a frequency of approximately 75 MHz will be generated; at this, the laser light pulse passes the laser resonator in both directions, which in the instant example will correspond to a length of 4 m. For mode-locking, a loss is periodically introduced (with the resonator round trip frequency), so that the laser begins to pulse. This results in a peak power of the pulses that is substantially higher (amounting to 100 kW to 200 kW, e.g.) than the output power of the laser in cw operation (which is 150 mW to 300 mW, e.g.).
Basically, it can be differentiated between two types of mode-locking.
In active mode-locking, a periodic loss is introduced by means of an active element, a modulator, which is supplied with energy from the outside via a driver. Thus, the laser is forced to perform its laser activity in those time intervals in which there is a lower loss, whereas the laser can store energy in those time intervals in which there are high losses.
In passive mode-locking, the effect of an optical non-linearity in the resonator is utilized, i.e. an optically non-linear element is arranged in the path of the laser beam, and this non-linear element changes its optical properties, such as the transmission or reflectivity, proportionally to the intensity of the laser beam. As such a non-linear element or Kerr element, the laser crystal itself may, e.g., be used which, in combination with a linear loss element, forms a so-called saturatable absorber in which the total loss will become the lower the higher the intensity of the impacting laser light. By a fluctuation in the laser power, a pulse is generated which "sees" a substantially lower loss than does the laser in cw operation (cf. U.S. Pat. No. 5,079,772 A). The laser body (solid laser) consists of a non-linear material whose optical "thickness" varies with the field intensity distribution of the laser radiation. The non-linear index of refraction, e.g., is a function of the square of the field intensity, i.e. the laser beam whose spatial field intensity distribution may be considered to be like a Gaussian curve, effectively "sees" an element with an optical thickness that varies over its cross-section in the case of a laser crystal having plane-parallel faces. In this manner, a focusing lens results from a plane-parallel non-linearity.
This optical Kerr effect may be utilized for mode-locking in two manner (so-called "Kerr-lens mode-locking"): In the case of the so-called "soft" aperture (cf. Spence et al., Optics Letters, Jan. 1st, 1991, Vol. 16, p. 42-44), the pump beam (in Ti:S lasers the energy is supplied by means of green laser, such as, e.g., argon laser) is very much focused in the laser crystal so that the resonator beam produced by the Ti-S laser (approximately 800 nm, infrared) may then take up the greatest part of the pump energy, i.e. experience the highest gain, if it has the smallest diameter. Thus, the higher the intensity or the field strength of the pulse, the more the laser pulse will be focused and the greater its gain at any passage through the laser crystal, whereby its intensity is increased again. This positive feedback leads to a stable mode-locking.
In the case of the so-called "hard" aperture (cf. e.g. U.S. Pat. No. 5,079,772 A and U.S. Pat. No. 5,097,471 A) the effect that an aperture restricts the resonator beam at a site where it has a larger diameter at that time when the intensity (field strength) is lower, and has a smaller diameter at that time when the intensity is higher and the resonator beam thus is focused in the laser crystal. The invention particularly relates to such a laser arrangement, i.e. one having a "hard" aperture.